1. Field of the Invention
The present invention pertains generally to the field of chemical sensors, more particularly, to the field of chemical sensors based on indicator dyes.
2. Background of the Invention
Optical sensors of gaseous pollutants in air based on indicator dyes have recently become fast expanding technology (see References 1 and 2). There are some features of these sensors that attract potential users: simplicity, compactness, ruggedness, robustness, and tolerance of electro-magnetic interference. The major problem of these sensors has been their poor sensitivity. Attempts to address this problem are often based on increasing the length of interaction of light with medium containing indicator dye. Longer interaction length is usually achieved by letting the light to pass through a light guiding structure either filled with the indicator dye or having an optical contact with the medium containing the dye.
In Reference 3, Klein et al. teach that a chemical sensor can be made as a strip multi-mode waveguide fabricated by a field-assisted ion-exchange in B-270 glass spin-coated with a porous sol-gel layer that has an immobilized indicator dye (bromocresol purple) sensitive to ammonia Light from two light-emitting diodes, at 600 and 700 nm, was sent through a standard gradient-index optical fiber. The fiber was butt-coupled with the waveguide at one of its ends. The other flat end was coated with a reflective aluminum coating. After passing through the waveguide, the light was reflected by the aluminum mirror back into the fiber. The intensity of light at each wavelength (600 and 700 nm) was modulated at its own distinguished frequency. After receiving light back from the fiber, a lock-in amplifier was used to extract signals proportional to the intensity at 600 and 700 nm respectively. At the presence of ammonia, only light at 600 nm experienced drop in intensity due to the reversible increase of absorption of the indicator dye exposed to ammonia. Division of the signal proportional to 600-nm intensity by the 700-nm signal with an electronic divider eventually produced an output signal that is proportional to the concentration of ammonia. The process of fabrication of the waveguide is complex and costly. Coupling of the multi-mode fiber with the waveguide requires special alignment and some means, not specified, of fixing the fiber to the waveguide.
In Reference 4, Caglar et al. teach that the chemical sensor can be made by attaching to the end of a plastic optic fiber a cluster of AMBERLITE XAD-7™ polymer micro-beads with an immobilized indicator dye, bromothymol blue. The light sent through the fiber is reflected back from the cluster. The intensity of the reflected light decreases when the cluster is exposed to ammonia thus producing a sensing effect. However, the size of the region where the reflected light interacts with the exposed beads is very short, a few microns. The sensitivity of the sensor is relatively poor.
In References 5 and 6, Hartman et al. and Bowman et al. teach that the chemical sensor can monitor changes in the refractive index of a polymer where the sensor can be made as a planar slab waveguide with gratings as means of coupling light with the waveguide. The polymer is poly(vinyl alcohol), polyimide CIBA-Geigy Probimide 285, dimethylsiloxate bis-phenol copolymer PS254 from Petrarch Systems Inc., and hard silicone OF20 from Shin-Etsu Chemical Co. Fabrication of gratings requires rather costly photolithographic process to be used. Change of the refractive index of the light guiding layer due to exposure to an analyte or due to variation of temperature changes the efficiency of coupling thus affecting the reading of the sensor.
In Reference 7, Lieberman et al. teach that a sensor of carbon monoxide can be made using a tip of a specially processed dye-doped porous optic fiber (multimode porous silica fiber, 2-8-nm pore diameter, 2 cm in length, and 250 micron in diameter) as a sensing element. This sensor requires a special, rather complex, procedure of preparation of the porous fiber and filling it with sensitive dye. The length of interaction of light with the sensitive material cannot be made large due to effects of scattering. The sensitivity of the sensor remains poor.
In Reference 8, Qi et al. teach that the sensor of ammonia can be made using a layer of indicator dye bromothymol blue deposited by vacuum evaporation on an ion-exchanged glass waveguide. However, conventional prism couplers are mechanically attached to the waveguide from the side of the ambient air. This makes the couplers and other optical elements open to the possible harmful effects of ammonia as well as to dust and atmospheric moisture.
In U.S. Pat. No. 4,513,087, Giuliani et al. describe a sensor wherein the principle of operation is based on transmitting light through an optical waveguide coated with an oxazine perchlorate dye film whose optical absorption between 500 and 700 nm changes from high to low when exposed to ammonia, hydrazine, or pyridine and returns to its original high level when the chemical is removed. The sensor consists of an optical waveguide made from a glass capillary tube with two flat ends. The outer wall of the tube is spray-coated (from a solution) with an oxazine perchlorate dye film. The tube is surrounded by a cell with inlet and outlet for letting the gas in and out. A light source, including a light-emitting diode (LED) and a flasher connected to LED, is optically “butt” coupled to one end of the tube. A light sensor, including a phototransistor, is optically butt coupled to another end of the tube to receive the light pulses and convert them to electrical pulses. Butt coupling to the tube is simply achieved by putting LED and phototransistor in mechanical contact with the tube. Coupling is simple and requires no alignment. Coupling efficiency is less sensitive to moderate mechanical vibrations. An amplifier is connected to the light sensor for amplifying the electrical pulses. A rectifier is connected to the amplifier to rectify the amplified pulses. A filter is connected to the rectifier to smoothen an output signal from rectifier. An indication device (chart recorder) is connected to the filter for showing the concentration of the gas passing through the cell.
The disadvantages of the Giuliani sensor are:
1) Sensitivity is poor.
2) The sensitive coating is sensitive to atmospheric moisture: the reading of the sensor fluctuates when the relative humidity changes.
3) The sensor is open to the noise produced by the slow fluctuations of the intensity of the light source and the noise produced by rapid fluctuations of ambient light and by the photodetector.
4) The sensor lacks means of self-calibration for achieving high accuracy.
All the proposed solutions to the problem of poor sensitivity rely on some sort of light guiding structure, planar waveguide or optical fiber, which requires rather complex techniques of coupling light with the structure that makes sensor expensive and vulnerable to mechanical vibrations. Besides that, dependence of the response of the sensor to atmospheric moisture, fluctuations of the intensity of the light source in combination with the noise produced by ambient light and photodetector, and the lack on any means of self-calibration have to be addressed.
To summarize, there is a great need for an inexpensive chemical sensor with the following characteristics:
1) a higher sensitivity should be achieved with a simple and reliable technique resistant to contamination and tolerant of a wide temperature range;
2) the effect of atmospheric moisture should be reduced;
3) the effects of noise originating from the fluctuations of the intensity of the light source as well as the noise produced by ambient light and the photodetector should be reduced; and
4) a self-calibration feature should be provided.